Process for growing a film epitaxially upon an oxide surface and structures formed with the process

ABSTRACT

A process and structure wherein a film comprised of a perovskite or a spinel is built epitaxially upon a surface, such as an alkaline earth oxide surface, involves the epitaxial build up of alternating constituent metal oxide planes of the perovskite or spinel. The first layer of metal oxide built upon the surface includes a metal element which provides a small cation in the crystalline structure of the perovskite or spinel, and the second layer of metal oxide built upon the surface includes a metal element which provides a large cation in the crystalline structure of the perovskite or spinel. The layering sequence involved in the film build up reduces problems which would otherwise result from the interfacial electrostatics at the first atomic layers, and these oxides can be stabilized as commensurate thin films at a unit cell thickness or grown with high crystal quality to thicknesses of 0.5-0.7 μm for optical device applications.

This application is a continuation-in-part of application Ser. No.08/100,743 filed Jul. 30, 1993 and entitled PROCESS FOR GROWING A FILMEPITAXIALLY UPON AN MgO SURFACE AND STRUCTURES FORMED WITH THE PROCESS,the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

This invention relates generally to the preparation of structures foruse in a semiconductor and/or optical wave guide applications andrelates, more particularly, to the growth of an epitaxial film uponsurfaces, such as an alkaline earth oxide surface.

Oxides in a class of oxides known as perovskites and spinels are knownto exhibit technologically-significant properties, such asferroelectricity, ferromagnetism, piezoelectricity, superconductivityand nonlinear electro-optic behavior, and for this reason, are grownupon substrates for the purpose of incorporating these properties withinelectronic devices. With such oxides grown upon substrates, theaforementioned properties can be taken advantage of in a number ofdevices, and in particular, are believed to be well-suited for use inFaraday Rotators for optical isolators and in magnetic memoryapplications.

Of these electronic devices, optical guided wave (OGW) devicesconstructed with perovskites are relatively demanding from thestandpoint of optical clarity and necessarily require long rangestructural coherence. Heretofore, the optical clarity and structuralcoherence of a perovskite film grown upon an alkaline earth oxide, suchas MgO, has been limited due, at least in part, to the inability to growa perovskite upon the alkaline earth wherein the grown perovskite is ofa single orientation. It would be desirable to provide a process forgrowing perovskite of single-orientation upon an alkaline earth oxideand thus enhance the quality of the resulting structure for OGWapplications.

Accordingly, an object of the present invention is to provide a new andimproved process for growing a perovskite or a spinel of singleorientation on an alkaline earth oxide and structures formed with theprocess.

Another object of the present invention is to provide such a processwhich is well-suited for coating an alkaline earth oxide surface with asingle layer of a Group IVA element oxide, i.e. TiO₂, ZrO₂ or HfO₂.

Still another object of the present invention is to provide such astructure which is well-suited for use in an OGW applications or forincorporation within an integrated circuit.

Yet another object is to provide a new and improved process for growinga perovskite or a spinel or constituents of a perovskite or spinelspitaxially upon a surface provided by a Group IVA element oxide or anoxide constituent of a perovskite or a spinel and structures formed withthe process.

A further object of the present invention is to provide such a structurewhose ferromagnetic properties render it well-suited for use inmagneto-optic applications.

SUMMARY OF THE INVENTION

This invention resides in a process for coating a body with an spitaxialfilm wherein the body has a surface provided by one of an alkaline earthoxide, a Group IVA element oxide, an oxide constituent of a perovskiteand an oxide constituent of a spinel and structures formed with theprocess.

One embodiment of the process includes the steps of growing, bymolecular beam epitaxy (MBE) techniques, a single plane of metal oxidehaving a metal element of a group of metals consisting of Ti, Zr, Hf, V,Cr, Mn, Fe, Co, Ni and Cu upon a surface provided by an alkaline earthoxide so that the metal and oxygen atoms of the single plane aredisposed at ordered sites across the alkaline earth oxide surface. In afurther embodiment of the method, the step of growing a single plane ofmetal oxide of the aforementioned group of oxides is followed by thestep of growing, by MBE techniques, a constituent metal oxide plane of aperovskite or a spinel upon the single plane of metal oxide wherein themetal of the constituent metal oxide plane provides the large cation inthe perovskite or spinel crystalline structure.

In another embodiment of the process, the body upon which an epitaxialfilm is coated has a surface defined by metal oxide provided by either aGroup IVA element oxide or an oxide constituent of a perovskite orspinel crystal wherein the metal element of the metal oxide provides arelatively small cation in the crystalline form of the metal oxide andthe metal and oxygen atoms of the metal oxide are disposed at orderedsites across the oxide surface. This process embodiment includes thesteps of growing, by MBE techniques, a constituent metal oxide plane ofa perovskite crystal or a spinel crystal epitaxially upon the singleplane of metal oxide wherein the metal element of the constituent metaloxide plane provides a relatively large cation in the perovskite orspinel crystalline structure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a body upon which an epitaxialperovskite or spinel can be grown in accordance with an embodiment ofthe method of the present invention.

FIG. 2 is an exploded perspective view of a structure within which afilm of perovskite is grown upon a layer of MgO and illustratingschematically the successive layers of constituents comprising thestructure.

FIG. 3 is a schematic perspective view of ultra high vacuum equipmentwith which steps of the present invention may be performed.

FIG. 4 is a SEM micrograph image of a cross section of a BaTiO₃ film of0.6 μm thickness epitaxially grown upon MgO(001).

FIG. 5 is a cube model representing the lattice orientation at theinterface of a structure wherein an MgO surface is covered with BaO.

FIG. 6a is a photograph providing RHEED data for a clean MgO surfacewherein the data is obtained along a [100] zone axis.

FIG. 6b is a photograph providing RHEED data for a single layer coverageof BaO on (001)MgO wherein the data is obtained along a [100] zone axis.

FIG. 7a is a photograph (like that of FIG. 6a) providing RHEED data fora clean MgO surface wherein the data is obtained along a [100] zoneaxis.

FIG. 7b is a photograph providing RHEED data for one monolayer coverageof TiO₂ on MgO(001) wherein the data is obtained along the [100] zoneaxis.

FIG. 8a is a plan view of a ball model of a clean MgO surface.

FIG. 8b is a plan view of a ball model of a one monolayer coverage ofTiO₂ on MgO(001).

FIG. 9 is a table providing in-plane and out-of-plane structure data andindex of refraction data for SrTiO₃ and BaTiO₃ thin films on MgO.

FIG. 10 is a graph providing data relating to wavelength dependence ofoptical loss in thin film SrTiO₃ on MgO.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Turning now to the drawings in greater detail, there is shown in FIG. 1a body or wafer 20 having a surface 22 defined by substrate layer of analkaline earth oxide, i.e. the (001) face, upon which a perovskite orspinel of single-orientation can be grown. In the interests of thepresent invention, the surface layer of the alkaline earth oxide can beprovided by the outer layer of a body comprised entirely of the alkalineearth oxide or the outer layer of a series of layers formed upon a basesubstrate comprised, for example, of a semi-conducting material such assilicon. In either instance, however, the crystalline structure of thealkaline earth oxide is clean, ordered and atomically smooth to promotethe subsequent epitaxial growth thereupon of constituents of aperovskite crystal.

The crystalline lattice structure of perovskite is face-centered-cubic(fcc) and includes a plane of a Group IVA element oxide, i.e. an oxideof a group consisting of TiO₂, ZrO₂, and HfO₂, and another plane of adifferent metal oxide. For example, the crystalline lattice structure ofthe perovskite BaTiO₃ includes a plane of TiO₂ and a plane of BaO.Similarly, the bulk crystalline structure of the perovskite SrTiO₃includes a plane of TiO₂ and a plane of SrO. As will be apparent, anembodiment of the process of the invention described herein involves theinitial formation of a plane of a Group IVA element oxide upon thealkaline earth oxide surface and the subsequent formation of additionalplanes of metal oxide and a Group IVA element oxide upon the initialplane of the Group IVA element oxide so that the subsequently-formedplanes alternate with one another.

As will be apparent herein, the crystalline lattice structure of anoxide in the oxide class known as spinel is comparable to thecrystalline lattice structure of a perovskite, i.e. is aface-centered-cubic, in a manner which renders the present inventionapplicable to the growth of spinels, as well as perovskites.

With reference to FIG. 2, there is illustrated an exemplary structure,indicated 24, upon which alternating planes 26 and 28 of the Group IVAelement oxide TiO₂ and metal oxide, respectively, are formed upon thealkaline earth oxide surface 22 comprised, in this instance, of MgO.Each plane 26 or 28 is formed upon the MgO surface 22 by molecular beamepitaxy (MBE) techniques and with MBE equipment. Briefly, the MBEequipment with which the process described herein can be carried outincludes an ultra high vacuum (UHV) growth/characterization facility, afragment of which is indicated 30 in FIG. 3. The facility 30 includes acontainer 32 having an inner chamber within which the body 20 ispositioned so that its surface 22 faces downwardly, and a plurality ofcanisters 34, 36 and 38 are provided within the base of the container 32for providing a vapor source of metal desired to be added to thesubstrate surface during the formation of the structure 24. In thisconnection, each canister 34, 36 and 38 is adapted to hold a cruciblecontaining a desired metal, and in this case, the canisters hold metalconstituents of the perovskite, e.g., BaTiO₃, SrTiO₃, CaTiO₃ or MgTiO₃,desired to be formed upon the MgO surface 24.

An opening is provided in the top of each canister, and a shutter isassociated with the canister opening for movement between a closedcondition at which the interior of the canister is closed and therebyisolated from the MgO surface 22 and a closed condition at which thecontents of the container 32, i.e., the metal vapor, is exposed to theMgO surface 22. In addition, an oxygen source 40 is connected to thechamber so that by opening and closing a valve associated with thesource 40, oxygen can be delivered to or shut off from the chamber. Theopening and closing of each canister shutter and the oxygen source valveis accurately controlled by a computer controller (not shown).

Before the desired layers, or planes, are grown upon the MgO surface 22,the MgO surface is rendered atomically smooth. To this end, the MgOsurface 22 can be treated with a polishing compound which iscommercially available as a cleaner under the trade designation Syton.The body 20 is then placed within the UHV facility 30, and thetemperature of the body 20 is raised to about 1000° C. At this elevatedtemperature, unwanted contaminants, such as water and dirt, are drivenfrom the surface 22 and Mg ions which may be under strain at the surface22 are permitted to shift to a more stable, or relieved, position. Whilemaintaining suitable control over the operation of the MBE facility 30,MgO is grown onto the surface 22 to restore crystalline perfection atthe MgO surface as MgO is deposited within so as to fill voids orsimilar defects which may exist across the surface 22. By growing anadditional thickness of about 1000 Å of Mg onto the surface 22, thedesired cleanliness and smoothness of the surface 22 is obtained.

In preparation of the growth of TiO₂ onto the MgO surface 22, thepressure in the UVH chamber is lowered to between about 2-5×10⁻⁷ torr.The desired layer of TiO₂ is then built upon the MgO surface 22 byconventional MBE techniques while the chamber pressure is maintainedbetween about 2-5×10⁻⁷ torr. For example, Ti metal vapor could initiallybe deposited upon the MgO surface 22 and then oxygen from the source 40could be released over the surface so that the desired layer of TiO₂ isformed at the surface 22. Alternatively, the surface 22 could besimultaneously exposed to Ti vapor and oxygen, in controlled amounts, sothat TiO₂ forms and then accumulates on the surface 22.

During either of the aforementioned deposition processes involving theTiO₂ layer, careful control of the MBE operation is maintained to ensurethat no more than one layer, i.e., one plane, of TiO₂ is deposited uponthe surface 22. The bulk form of the compound TiO₂, as characterized bythe ordered surface structure formed in this step, has a nonequilibriumstructure and is not found in nature, and there exists a tendency forthe formed TiO₂ to accumulate into clusters if the surface 22 is exposedto a greater amount of TiO₂ than is needed to comprise a single plane ofTiO₂. Of course, if such clusters develop, the TiO₂ layer looses itsorder, and the ability to grow ordered layers upon the TiO₂ layer isdestroyed. Thus, careful control must be maintained over the depositionof Ti vapor and the release of oxygen from the source 40 so that asingle layer, and only a single layer, of TiO₂ accumulates at orderedsites upon the MgO surface 22.

Following the development of the desired layer of TiO₂ upon the MgOsurface 22, a layer of metal oxide which comprises the other plane ofthe desired perovskite is formed upon the TiO₂ layer. If, for example,the desired perovskite is BaTiO₃, then the vapor released in thefacility chamber is Ba, and if the desired perovskite is SrTiO₃, thenthe vapor released into the chamber facility is St.

Conventional MBE techniques are used to grow the desired oxide, e.g.,BaO or SrO, layer upon the formed TiO₂ layer. For example, the metalvapor, e.g., Ba or Sr, may be initially deposited upon the TiO₂ surface,and then the oxygen may be subsequently released into the chamber sothat the metal oxide forms upon the TiO₂ surface. Alternatively, theTiO₂ layer could be simultaneously exposed to metal vapor and oxygen sothat the metal oxide accumulates on the TiO₂ layer. In either event,careful control should be maintained over the deposition operation hereso that no more than one plane of the desired metal oxide is developedat this stage upon the TiO₂ layer and so that the pattern of metal oxidedeposited upon the TiO₃ layer is ordered.

Upon formation of the desired plane of metal oxide, a second plane ofTiO₂ is grown upon the metal oxide plane in accordance with theaforedescribed techniques used to grow TiO₂ onto the MgO surface. Then,upon formation of the desired second plane of TiO₂, a second plane ofthe metal oxide, e.g., BaO or SrO, is grown upon the second plane ofTiO₂.

Thereafter, layers of TiO₂ and metal oxide are formed in an alternatingfashion until at least about twenty-five cell units of the desiredperovskite are grown upon the MgO surface. Dislocations which maydevelop within the formed layers nucleate so as to provide internalstrain relief within the first twenty-five cell units so that latticestrain does not appear at the surface of the layup of planes. Thus, thesurface defined by the twenty-fifth cell unit is ordered and free ofstrain.

Once the strain-free surface of perovskite is formed, steps can then betaken to grow addition layers of the perovskite upon the build up ofcell units. In this connection, subsequent growth of the perovskite uponits strain-free bulk form is homoepitaxial, rather than heteroepitaxialso that the characteristics of the interface between adjacent layers ofTiO₂ and metal oxide are not likely to present problems during growth.Thus, the perovskite can be built upon itself after the initialtwenty-five cell units of perovskite are formed. To this end, theperovskite is grown layer-by-layer upon the strain-free surface byconventional MBE techniques to that each layer of perovskite is one cellunit high. For example, the strain free surface may be initially beexposed to Ti and metal, e.g., Ba or Sr, vapors and then to oxygen sothat the perovskites forms upon the strain-free surface. Alternatively,the strain-free surface can be exposed simultaneously to the Ti andmetal vapors and oxygen so that the perovskite forms and then settlesupon the strain-free surface. In either instance, careful control of theMBE process is maintained so that the build up of successive layers ofthe perovskite is effected epitaxially.

The clarity of the resulting perovskite is realized, at least in part,by the aforedescribed build up of alternating layers of TiO₂ and metaloxide on the MgO surface in that this build up minimizes undersirableeffects which could otherwise result from interfacial electrostaticsdeveloped between MgO and the superposed layers subsequently grownthereon. To appreciate the interfacial electrostatics issue, thestructure of the perovskite oxides can be considered. The distinguishingcharacteristic of the perovskite oxide class is recognized as aclosest-packing of large cations and oxygen anions arranged as stackedsheets normal to a [111] direction. The octahedral interstices that formas a result of this sheet-stacking sequence are in turn filled withhigher valence, smaller cations. The resulting structures are cubic withlow index stable crystal faces. The naturally occuring crystaltruncations are {001} and are then, for example with BaTiO₃, either BaOplanes or TiO₂ planes, as mentioned earlier. The ion sizes and chargesin these planes are distinctly different, and the initiation of aheteroepitaxial growth sequence for such a structure on anotherinsulating oxide must take this into account.

With reference to the micrograph image of FIG. 4, there is shown afracture cross section of a representative BaTiO₃ film on (100)MgO. TheFIG. 4 material was grown by using source-shuttering MBE techniques inultra high vacuum. The film is adherent, single phase and opticallyclear. The epitaxy is cube-on-cube and uniquely results from theaforedescribed layering sequence that begins at the TiO₂ -plane of theperovskite structure. The layering sequence is a requirement forsingle-orientation, epitaxial growth of a perovskite on MgO.

For a heteroepitaxial transition between insulating oxides, theinterface electrostatics (ion--ion near neighbor interactions) of thefirst layers critically determine whether a commensurate structure candevelop. For example, in going from MgO to BaTiO₃ on the (001) face ofMgO, if the transition is initiated at a barium oxide plane, thestructure at the interface cannot develop commensurately with the MgOsurface. The basic incompatibility results from the large ion-sizedifference between barium and magnesium. In particular, it is impossibleto avoid near-neighbor ion configurations where cation--cation oranion--anion repulsive interactions occur in large numbers. Thisnaturally leads to interfacial energy and an inherent instability. Ineach study made up until now which has been directed to interfacialequilibrium and surface segregation phenomena for the alkaline earthoxides, the clear result emerged that no single layer of BaO on MgOexisted that was energically stable. We have found that the energeticstability is of paramount importance to the growth of single-orientationperovskites on MgO.

For purposes of comparison, barium metal and oxygen was deposited onto aMgO surface at a substrate temperature of 500° C. to form BaO at a 1/2monolayer coverage based on the MgO surface. This monolayer coverage isequivalent to one monolayer of BaO in BaTiO₃. The high interfacialenergies that would result from commensurate BaO epitaxy on MgO shoulddrive some mechanism for lowering the interfacial energy. In thisregard, there is shown in FIG. 5 a cube model of the interface andassociated reflection high-energy electron diffraction (RHEED) patternsfrom clean and 1/2 monolayer BaO-covered (100)MgO surfaces. Theimplication of surface segregation theories is that island-likenucleation of incommensurate BaO-type structures should develop, and itis believed that this does occur. The cube model shown in FIG. 5 showsan idealization of parallel and 45°-rotated morphologies of an (100)interface between MgO and BaO, and FIGS. 6a and 6b show diffractionpatterns as experimental confirmation of their existence. The RHEEDpattern shown in FIG. 6a results from an MgO surface prepared in the MBEsystem by growing 100 nm of MgO homoepitaxially on (001)MgO. The 0,0 andallowed 0,2 surface rods are seen. In FIG. 6b, surface diffraction atthe same zone axis is illustrated but is modified by asingle-layer-coverage BaO deposition. It can be seen in FIG. 6b thatincommensurate crystallite orientations have formed and give rise todiffraction at what would be the 0,2 rod position for cube-on-cube BaOand at the 1,1 rod of 45°-rotated BaO as well. Moreover, in addition tothe rod spacing indicating the microstructural characteristics of theinterface, the diffraction intensity is modulated along the reciprocallattice BaO rods in a Bragg-like manner, i.e., 3-dimensional diffractionoccurs that is indicative of "islanding" or surface roughening. Thesemulti-orientation, 3-dimensional island structures defeat any attempt atgrowing optical-quality, thick perovskite films.

With reference again to the construction of the structure of the presentinvention, there are provided in FIGS. 7a and 7b photographs of RHEEDdata which illustrate the dramatically different result that can beobtained by moving up one plane from the MgO layer (whose ball model isdepicted in FIG. 8a) in the BaTiO₃ unit cell to the TiO₂ plane (whoseball model is depicted in FIG. 8b) and initiating the growth sequence atthat point. A commensurate, atomically flat layer of TiO₂ can form inwhich every other cation row is vacant over the underlying Mg²⁺ sites.This TiO₂ surface satisfies the electrostatic requirements foranion-cation near-neighbor pairs at the interface and is a low-energy,stable truncation of the MgO surface. The missing row of cations in thislayer provides the energetically favorable sites for subsequent bariumion attachment to the crystal surface. As the perovskite growth iscontinued with alternating barium and titanium deposition cycles, BaTiO₃grows layer-by-layer and strain relief can occur by nucleation of simpleedge dislocations maintaining the single orientation cube-on-cubeepitaxy. The BaTiO₃ lattice parameter relaxes to its strain-free, bulkvalue within ten unit cells from the original interface. The transitionfrom heteropitaxy to homoepitaxy of the perovskite is completed with thedesired single-orientation material and its advantageous long-rangestructural coherence. With the transition from heteroepitaxy tohomoepitaxy accomplished in the manner described above, growth rates onthe order of 1 μm/hr can be attained at temperatures as low as 500° C.by codeposition of barium and titanium or strontium and titanium withoxygen arrival rates equivalent to pressures of 10⁻⁷ torr. Structuraland optical characteristics of the resulting thin films are provided intable form in FIG. 9.

The MBE process described above for the stabilization of the interfacebetween a perovskite oxide and the alkaline earth oxide MgO provides anopportunity heretofore unavailable to exploit the electro-opticproperties of thin-film epitaxial ferroelectrics in waveguideapplications. In support of this contention, there is provided in FIG.10 a plot of the waveguide dependence for optical loss in thin filmSrTiO₃ on an MgO surface. Such a film is of high optical clarity and canbe directly compared with the performance of LiNbO₃, the most commonlyapplied material in EO devices. It is believed that this is the firstdemonstration of such optical clarity of SrTiO₃ and BaTiO₃ grown in thinfilm form. The crystal quality that is obtained by the methods describedabove does not result from incremental improvements upon knowninformation, but rather, is attained by directly addressing thefundamental requirements of interfacial energy minimization betweenperovskite and alkaline earth oxides.

It will be understood that numerous modifications and substitutions canbe had to the aforedescribed embodiments without departing from thespirit of the invention. For example, although the aforedescribedprocess describes a build up of a relatively thick film of perovskiteupon a MgO surface, a usable product which could, for example, permitthe intrinsic properties of MgO to be studied may include only a singlelayer of TiO₂ overlying a MgO surface. Thus, in accordance with thebroader aspects of the present invention, an embodiment of the processcould terminate upon the formation of a single plane of TiO₂ (or anotherGroup IVA element oxide) upon a MgO (or other alkaline earth oxide)surface.

Still further, although the aforementioned embodiments have beendescribed in connection with perovskites which include a plane oftitanium oxide (TiO₂), the principles of the present invention areapplicable to other perovskites and oxides in the class of oxides knownas spinels. The distinguishing structural characteristic of theperovskite or spinel oxide class with which this invention is concernedis recognized as a closest-packing of large cations and oxygens arrangedas stacked sheets, and between these sheets are positioned highervalence, smaller cations. For example, in each of the perovskitesBaZrO₃, SrZrO₃ and PbZrO₃, the metal zirconium provides the smallcations in the crystalline structure (and bonds with oxygen in one planeof the structure to form ZrO₃) while the metal element Ba, Sr or Pbprovides the larger cations. Similarly, in the perovskite SrHfO₃, themetal hafnium plays the role of the small cations while the metalstrontium plays the role of the large cations. Along these lines, themetal oxide plane of a perovskite crystal containing the small cationcan be comprised of a mixture of different, albeit suitable, e.g. GroupIVA, elements. For example, the perovskite BaTi_(x) Zr_(1-x) O₃ can bebuilt epitaxially upon a substrate of MgO (or another alkaline earthoxide) in accordance with the principles of the present inventionwherein titanium and zirconium are used in the construction of thecrystalline planes of the perovskite structure which include the smallcations. The perovskites are generically in the stochiometry of ABO₃wherein A is an element like Mg, Ba, Sr, Ca and Pb, all of which havevalence states of +2, and B is an element like Ti, Hf or Zr havingvalence states of +4.

Similarly, the crystal structure of an oxide known as a spinel is knownto include a face whose lattice structure, when viewed frontally,simulates that of the crystalline form of a Group IVA oxide (see, e.g.the ball model of TiO₂ depicted in FIG. 8b). In other words, thesespinel oxides are provided with a constituent oxide plane wherein themetal element of the oxide in the plane provides a relatively smallcation with respect to the size of the oxygen in the crystalline form ofthe oxide and the metal and the oxygen atoms of the metal oxide aredisposed at ordered sites across the oxide surface. The spinel oxidesare provided with a second constituent oxide plane wherein the metalelement of the oxide in this second plane provides a relatively largecation in the crystalline form of the oxide. The spinels are genericallyin the stochiometry of A₂ BO₄ where A is an element, i.e. a large cationelement, that is not magnetic, such as Mg, Ba, Sr, Ca and Pb. Theseelements all have filled outer shell electron configurations so thatthere are no unpaired electrons that give rise to permanent magneticmoments. B is an element, i.e. a small cation element, that can bemagnetic, such as Ti, V, Cr, Mn, Fe, Co, Ni and Cu. Theselatter-mentioned elements come from the transition or rare earth elementseries and have unfilled inner electron shells containing unpairedelectrons which are then responsible for their permanent magneticmoments. The magnetic moments associated with these "B" elements undergoorder/disorder phenomena associated with ferromagnetic phasetransformations and then exhibit magneto-optic properties. Theseproperties can be taken advantage of in a number of devices, and inparticular, are believed to be well-suited for use in Faraday Rotatorsfor optical isolators and in magnetic memory applications.

It is believed that due to the aforediscussed similarity in thecrystalline forms of the Group IVA element oxides, the perovskites andthe spinels, a perovskite, a spinel or a constituent oxide plane of aperovskite or a spinel can be grown upon a surface provided by either ofthe Group IVA element oxides or an oxide constituent of a perovskite orspinel in accordance with the principles of the present invention. Tothis end, MBE techniques are used to grow an initial constituent planeof a perovskite or spinel crystal epitaxially upon the metal oxidewherein the metal element of the constituent oxide plane provides alarge cation in the perovskite or spinel structure. The build up ofepitaxial layers can then be continued (e.g. toward the formation ofperovskite in bulk or spinel in bulk) by growing, with MBE techniques, asecond epitaxial layer upon the initial layer wherein the secondepitaxial layer is comprised of a constituent metal oxide plane of theperovskite or spinel wherein the constituent metal oxide plane of thesecond epitaxial layer includes the metal element which provides thesmall cation in the perovskite or spinel crystalline structure.

Accordingly, the aforedescribed embodiments are intended for the purposeof illustration and not as limitation.

We claim:
 1. A process for coating a body with an epitaxial film whereinthe body has a surface provided by an alkaline earth oxide, the processcomprising the steps of:growing, by molecular beam epitaxy (MBE)techniques, a single plane of metal oxide having oxygen and a metalelement of a group of metals consisting of Ti, Zr, Hf, V, Cr, Mn, Fe,Co, Ni and Cu upon the alkaline earth oxide surface wherein the oxygenand metal atoms of the metal oxide are deposited upon the alkaline earthoxide surface in quantities sufficient to construct a single plane ofthe metal oxide and come to rest during the deposition process atordered sites across the alkaline earth oxide surface.
 2. The process asdefined in claim 1 wherein the metal element of the single metal oxideplane provides a relatively small cation with respect to the size of theoxygen in the crystalline form of the metal oxide and the step ofgrowing the single plane of the metal oxide is followed by a stepofgrowing, by molecular beam epitaxy (MBE) techniques, a constituentmetal oxide plane of a perovskite crystal or a spinel crystalepitaxially upon the single plane of metal oxide wherein the perovskiteor spinel has a crystalline form comprised of two metal oxide planes,and wherein the metal oxide of one of the two metal oxide planes of theperovskite or spinel crystalline form includes one metal which providesa small cation in the perovskite or spinel crystalline structure and themetal oxide of the other of the two metal oxide planes of the perovskiteor spinel crystalline form includes another metal which provides a largecation in the perovskite or spinel crystalline structure, and whereinthe constituent metal oxide plane grown by this step includes the metalwhich provides the large cation in the perovskite or spinel crystallinestructure.
 3. The process as defined in claim 2 wherein the constituentmetal oxide plane provides a first epitaxial layer and the step ofgrowing the first layer is followed by the step of:growing, by MBEtechniques, a second epitaxial layer upon the first epitaxial layerwherein the second epitaxial layer is comprised of a constituent metaloxide plane of a perovskite crystal or a spinel crystal wherein theconstituent metal oxide plane of the second epitaxial layer includes themetal which provides the small cation in the perovskite or spinelcrystalline structure.
 4. A process as defined in claim 3 wherein themetal oxide of the second epitaxial layer is a metal oxide having ametal element of a group of metals consisting of Ti, Zr, Hf, V, Cr, Mn,Fe, Co, Ni and Cu.
 5. A process as defined in claim 3 wherein the stepof growing the second epitaxial layer is followed by the repeating insequence of:growing another constituent metal oxide plane of aperovskite crystal or a spinel crystal upon the second epitaxial layerwherein the metal of said another metal oxide plane provides the largecation in the perovskite or spinel crystalline structure, and thengrowing a further constituent metal oxide plane of a perovskite crystalor a spinel crystal upon said another constituent metal oxide planewherein the metal of another metal oxide plane provides the small cationin the perovskite or spinel crystalline structure.
 6. The process asdefined in claim 3 wherein the step of growing the second epitaxiallayer is followed by the sequential steps of:a) growing a singleepitaxial plane of metal oxide having a metal element of a group ofmetals consisting of Ti, Zr, Hf, V, Cr, Mn, Fe, Co, Ni and Cu upon thesecond epitaxial layer; and b) growing a single epitaxial plane of metaloxide directly upon the single epitaxial plane of metal oxide grown instep a) wherein the metal oxide plane grown in this step b) is aconstituent metal oxide plane of a perovskite crystal or a spinelcrystal and includes the metal which provides the small cation in theperovskite or spinel crystalline structure.
 7. The process as defined inclaim 6 wherein steps a) and b) are repeated, inclusively, until nolattice strain within the layup of planes appears at the surface of thelayup.
 8. The process as defined in claim 6 wherein steps a) and b) arerepeated, inclusively, until the total number of constituent metal oxideplanes grown by step a) is at least twenty-five.
 9. The process asdefined in claim 8 wherein the twenty-fifth plane of constituent metaloxide grown by step a) is followed by the steps of:growing, by MBEtechniques, layers of a perovskite or spinel upon the grown layup ofplanes wherein the perovskite or spinel layers are grown in alayer-by-layer fashion.
 10. A process for coating a body with anepitaxial film wherein the body has a surface defined by a metal oxideprovided by either a Group IVA element oxide or an oxide constituent ofa perovskite crystal or a spinel crystal wherein the metal element ofthe metal oxide provides a relatively small cation in the crystallineform of the metal oxide and the metal and oxygen atoms of the metaloxide are disposed at ordered sites across the oxide surface, theprocess comprising the steps of:growing, by molecular beam epitaxy (MBE)techniques, a constituent metal oxide plane of a perovskite crystal or aspinel crystal epitaxially upon the single plane of metal oxide whereinthe metal element of the constituent metal oxide plane provides arelatively large cation in the perovskite or spinel crystallinestructure.
 11. The process as defined in claim 10 wherein theconstituent metal oxide plane provides a first epitaxial layer and thestep of growing the first layer is followed by the step of:growing, byMBE techniques, a second epitaxial layer upon the first epitaxial layerwherein the second epitaxial layer is comprised of a constituent metaloxide plane of a perovskite crystal or a spinel crystal and includes themetal which provides a relatively small cation in the perovskite orspinel crystalline structure.
 12. A process as defined in claim 11wherein the step of growing the second epitaxial layer is followed bythe repeating in sequence the steps of:growing another constituent metaloxide plane of a perovskite crystal or a spinel crystal upon the secondepitaxial layer wherein the metal element of said another metal oxideplane provides the large cation in the perovskite or spinel crystallinestructure, and then growing a further constituent metal oxide plane of aperovskite or spinel crystal upon said another constituent metal oxideplane wherein the metal of said another metal oxide plane provides thesmall cation in the perovskite or spinel crystalline structure.
 13. Theprocess as defined in claim 11 wherein the step of growing the secondepitaxial layer is followed by the sequential steps of:a) growing asingle epitaxial plane of metal oxide having a metal element of a groupof metals consisting of Ti, Zr, Hf, V, Cr, Mn, Fe, Co, Ni and Cu uponthe second epitaxial layer; and b) growing a single epitaxial plane ofmetal oxide directly upon the single epitaxial plane of metal oxidegrown in step a) wherein the metal oxide plane grown in this step b) isa constituent metal oxide plane of a perovskite crystal or a spinelcrystal and includes the metal which provides the small cation in theperovskite or spinel crystalline structure.
 14. The process as definedin claim 13 wherein steps a) and b) are repeated, inclusively, until nolattice strain within the layup of planes appears at the surface of thelayup.
 15. The process as defined in claim 13 are repeated, inclusively,until the total number of constituent metal oxide planes grown by stepa) is at least twenty-five.
 16. The process as defined in claim 15wherein the twenty-fifth plane of constituent metal oxide grown by stepa) is followed by the steps of:growing, by MBE techniques, layers of aperovskite crystal or a spinel upon the grown layup of planes whereinthe perovskite or spinel layers are grown in a layer-by-layer fashion.